Digital communication receivers must sample an analog waveform and then reliably detect the sampled data. Signals arriving at a receiver are typically corrupted by intersymbol interference (ISI), crosstalk, echo, and other noise. In order to compensate for such channel distortions, communication receivers often employ well-known equalization techniques. For example, zero equalization or decision-feedback equalization (DFE) techniques (or both) are often employed. Such equalization techniques are widely-used for removing intersymbol interference and to improve the noise margin See, for example, R. Gitlin et al., Digital Communication Principles, (Plenum Press, 1992) and E. A. Lee and D. G. Messerschmitt, Digital Communications, (Kluwer Academic Press, 1988), each incorporated by reference herein.
It is often desirable to allow for the equalization components to adaptively respond to changes in channel characteristics or ambient conditions, such as temperature and humidity. Adaptation algorithms typically adapt their filter coefficients in accordance with the signal statistics or the signal spectrum. Equalization algorithms will typically converge on a set of filter coefficients that are often dependent on the channel. In many applications, the data pattern may change suddenly and the converged equalizer coefficients will diverge if the data pattern is not spectrally rich. Thus, a degradation of bit error performance may be experienced.
A receiver typically also includes a clock and data recovery (CDR) system to recover the clock and data from an incoming data stream. The CDR system generates a clock signal having substantially the same frequency and phase as the incoming signal, which is then used to sample the received signal and detect the transmitted data.
Transmitted data may not be sufficiently spectrally rich to provide valid equalization or CDR update results (or both). For example, equalization or CDR updates performed for one frequency, may not be valid for other frequencies. Existing CDR and equalizer adaptation techniques, however, do not detect signal statistics to qualify their updates. Thus, the CDR system can potentially generate jitter due to polarization of high density transitions followed by low density transitions and vise versa. Likewise, equalization updates in the presence of polarized signal transitions can drive equalization coefficients away from their optimal values.
A need therefore exists for methods and apparatus for CDR and equalization update qualification. A further need exists for CDR and equalization adaptation methods and apparatus that can detect pattern statistics, and selectively disable CDR and equalization adaptation in the presence of poor pattern statistics. Yet another aspect of the invention ensures rich tone signals for adapting equalization and timing recovery.